Alignment of the liquid crystal director is important for many liquid crystal applications. Most field effect liquid crystal displays (LCDs) today use alignment layers to achieve high contrast ratio and maximum brightness. Currently, the predominated alignment technique is mechanical rubbing in which a substrate is coated with a thin polymer film and then buffed with cloth to control the direction of liquid crystal on the substrate surface. Typically, a polyimide is employed as a liquid crystal alignment layer. The liquid crystal alignment process requires the deposition of a thin polymer layer of a polyimide precursor, for example polyamide acid, and then cured. The polymer surface is then rubbed in one uniform direction to give a homogeneous disposition. The rubbing process also generates the so-called “pre-tilt” angle. The pre-tilt angle is important in various optical effects and switching mechanisms of nematic devices is associate with the nature of molecular structure of the polyimide. The advantage of this method is that it is suitable for mass production. However, the rubbing method has certain drawbacks including the production of dust and electrostatic charges, which is particularly disadvantageous for manufacturing high performance liquid crystal displays. Thus, a non-contact alignment technology would be highly desirable for use in manufacturing future generations of large, high-resolution LCDs and other liquid crystal technology.
Yet anther method for forming an alignment layer on a substrate is deposition by an oblique angle through evaporation of SiO2 for homogeneous alignment. This deposition method has proven to be cumbersome and complicated in the mass production of high-resolution LCDs. With these restrictions, finding a new alignment technique which would enable the control of pretilt angle and spatial resolution would have a major impact on existing liquid crystal technologies as well as provide a means for realizing new liquid crystal devices.
Liquid-crystal photoalignment, another non-rubbing liquid-crystal alignment method, has been developed for fabricating LCDs, where the substrates are photo-buffed to generate surface anisotropy and pretilt angle. This technique avoids many drawbacks of the traditional rubbing technique for liquid crystal alignment, such as sample contamination and electrostatic charge generation. Typically, liquid crystal photoalignment technique uses films of photo-cross-linkable polymers cured with linearly polarized ultra-violet light. Photodimerization, appearing in such polymer films upon illumination with linearly polarized UV light, leads to the generation of surface anisotropy and unidirectional liquid crystal alignment. In this alignment technique, the induced liquid crystal alignment aging with time is a common problem, which normally causes an imaging sticking problem.
Another non-contact alignment method using a low-energy ion beam aligned liquid crystal with diamond-like carbon film has been developed. The mechanism of alignment is due to the order induced by exposing the inorganic or organic material to the ion beam by selectively destroying the randomly arranged aromatic rings of diamond-like carbon atoms. The products of these reactions change boundary conditions. Subsequent improvements related to using ion beam alignment with various alignment materials, gas, feed, etc. were also developed. Although low-pretilt liquid crystal alignment can be easily produced using the ion beam alignment, high-pretilt alignment is realized only for the limited class of materials with low reproducibility. In addition, liquid crystal alignment deteriorates because of aging from the destructive process.
Inkjet printing is a known process that is both cost effective and simple. Inkjet technology provides a low cost, high speed, non-contact, and environmentally friendly process. Recently, much effort has been invested in turning inkjet printing into a versatile tool for various industrial manufacturing processes in order to accurately deposit a minute quantity of material on a substrate. For example, Seiko Epson, a pioneer company in consumer inkjet printers, has been conducting extensive research into industrial inkjet applications with the goal of applying the company's proprietary MicroPiezo® print head technology to mass production. Most recently, in an attempt to save the cost in mass production of LCDs, a method of combining the inkjet printing of a polyimide layer and rubbing the surface for liquid crystal alignment has been reported. Although the inkjet printing offers some advantages including a smooth and uniform alignment layer, the required rubbing step still diminishes the contribution and causes the same drawbacks including the production of dust and electrostatic charges.
Another micro-rubbing technology, with atomic force microscopy (AFM) tip on the alignment layer has been studied. This direct writing method has proven to be good for lab-scale investigation but not suitable in mass production. The limitations of AFM micro-rubbed technique for LC alignment are speed and size of the alignment. In light of the forgoing, the problems described in the above alignment methods need to be overcome, and there thus is a need for a new and effective alignment method for use in for future generations of large, high-resolution LCDs and other liquid crystal technologies.